Electrical heating elements



w. a. MATHESON E AL 3,427,421

ELECTRICAL HEATING ELEMENTS Sheet FIG.I

INVENTORS N 0 S E H T A M 6 w R F W JAMES F. CLUNE THEODORE J. PRICENSKIFIG.3

RNEY

Feb. 11, 1969 Original Filed May 7 1965 ws. MATHESON ETAL 3,427,421

ELECTRICAL HEATING ELEMENTS Original Filed May '7, 1963 Feb. 11, 1969Sheet FIG.4

FIG.5

.IS N Mm w m E Cm MWPm MLJ: D mam w F 0 wk WJT m ATTORNEY United StatesPatent 3,427,421 ELECTRICAL HEATING ELEMENTS Wilfrid G. Matheson,Marblehead, James P. Clune, Danvers, and Theodore J. Pricenski, Ipswich,Mass., assignors to Sylvania Electric Products Inc., a corporation ofDelaware Original application May 7, 1963, Ser. No. 278,660, now PatentNo. 3,274,374, dated Sept. 20, 1966. Divided and this application Feb.25, 1966, Ser. No. 547,108 US. Cl. 219-1049 6 Claims Int. Cl. Hb 5/02ABSTRACT OF THE DISCLOSURE An induction heating furnace including awater cooled helical coil disposed about a generally-helical plexus inthe shape of a susceptor which is formed of a multiplicity ofintertwisted helical convolutions of refractory metal WII'CS.

This case is a division of Ser. No. 278,660 filed May 7, 1963 now PatentNo. 3,274,374.

This invention relates to refractory metal heating elements forelectrical furnaces and more particularly to heating elements which aresuitable for use as anodes or susceptors. Such elements are particularlyuseful in high temperature electric discharge furnaces or inductionheating furnaces respectively.

Anodes or susceptors are well known to the art as heating elements andcertain of them have been previously fabricated of refractory metals. Inthe past however, elements using refractory metals have been fabricatedfrom sheet materials or machined from heavy stock and when so fabricatedthey do not possess crystal structures having optimum characteristics toresist breakage or maintain their geometric form. But we have discoveredthat refractory metal wire does possess these characteristics and whenproperly fabricated, the elements may be used successfully as anodes forhigh temperature discharge furnaces or susceptors for inducting heatingfurnaces utilizing a vacuum and/ or inert or reducing atmospheres. Thisdiscovery has led to the achievement of long-lived, rugged heatingelements which can withstand temperatures as high as 2000 C. and oftenas high as 2500 to 3000 C. When using refractory metal wire, theelements can have substantially identical electrical characteristics asthose known in the prior art, but yet be structurally far superior totheir solid sheet counterparts.

High temperature electric discharge furnaces operate by forming athermionic electron emission from a cathode which surrounds the anode ofthis invention. When adequate temperature is reached, the ions insteadof being supplied from the heated surface can be supplied from acontrolled flow of gas into the discharge region between the anode thecathode or from direct emission in a vacuum. In gas containing furnaces,the is ionized by collision with the electrons and by controlling thenumber of ions so released the temperature may be regulated. Thepositive ions liberated tend to neutralize the space charge between thecathode and the bombardment surface. They also act as current carriers,as do the released electrons, increasing the current, but the carriersadded relatively few; by far the greater proportion of the current iscarried by initial thermionic discharge. The latter case issubstantially exclusively true in high vacuum furnaces. The discharge isdiffuse and tends to spread over the entire surface of the bombardmentanode, and since it is diffuse there is no concentration of the currenton a single spot to form an arc crater, whereby the anode would bevaporized to form a true arc. Heating of the cathode is preferablyinitiated by passing current through it directly, although it may beindirectly heated when desired. Heating of the cathode also occurs dueto positive ion bombardment and by radiation from the anode and henceafter the discharge has been maintained for a long enough period forequilibrium temperatures to establish themselves, the cathodetemperature is maintained partly by heat from these sources and thepower supplied to it directly can be reduced.

Induction heating has been extensively applied in industry for melting,heat treating, brazing and soldering, hot forming and other processingoperations. In such applications, the susceptor which is electricallyconductive, is generally heated directly by induced current when it isplaced in an electromagnetic field established by a high frequencycurrent flowing through a surrounding induction heating coil. Thearticle of work is placed within the susceptor and heated by radiationand in recent years the use of such susceptors has significantlyextended the application of induction heating. In its simplest form, asusceptor can be a metal tube having magnetic characteristics interposedbetween an electromagnetic coil and the article of work to be heated.The susceptor is then heated by the magnetic field established by theinduction coil but the former essentially shields the work within it sothat the element being treated can be heated primarily by radiation andor conduction from the heated susceptor. Not only can electricallyconductive elements be heated thusly, but also many nonconductivematerials such as ceramics and plastics.

The susceptors or anodes according to our invention can take a number ofdifferent forms and shapes. In the preferred embodiment, a number ofhelically convoluted wires (helical coils) each having generallysubstantially similar diameters are intertwisted together in such a waythat generally two convolutions are intertwisted in each otherconvolution. In this manner we continue the intertwisting until aforaminous plexus is formed. Preferably, the plexus is then bent aboutthe axes of the helical coils into a cylinder and welded at its abuttingends to form a seam. The diameter of the cylinder can vary withoutlimit, depending only upon the size of the furnace in which it is to beused. Large diameter wires can be used for larger cylinders and smallerdiameter wires for smaller cylinders if desired.

The use of a series of intertwisted helical convolutions allows greaterlatitude in the application of the anode or susceptor. Even afterheating and thus when each of the individual convolutions is quitebrittle, they are each movable in their adjacent convolutions and hencecan withstand stresses which would ordinarily fracture solid sheets ofsimilar refractory metals. Coupled with the inherent flexibility ofplexuses is the increased strength resulting from drawn wire over solidsheets due to the incorporation of long fiber-like crystals as a resultof processing. Thus even though the individual convolutions of wire maybecome quite brittle after the element has been used once, the plexusitself is still fairly flexible and can be moved in the furnace withoutan inordinate danger of breaking. Furthermore, the problem of thermalshock, usually resulting from heating too rapidly or cooling too quicklyis materially reduced due to the construction and crystal structureprovided by wire. When desired for greater stability, and whenflexibility can be dispensed with as a criteria, our plexuses may serveas a base for a thicker coating of flame sprayed refractory metals whichmay be prepared according to conventional techniques. The use of flamesprayed coatings is not generally practical with solid sheet anodes orsusceptors because flame sprayed material will not permanently adhere tosuch bases.

Accordingly, the primary object of this invention is the fabrication ofheating elements having increased mechanical strength and the ability towithstand fairly large thermal shocks without breakage.

Another object of this invention is the substantial elimination ofdistortion in the geometrical shape of anodes and susceptors after theirelevation to high temperatures in furnaces.

Another object of this invention is the extension of the life ofelectrical heating elements fabricated from refractory metals which canbe heated to temperatures as high as about 3000 C.

A feature of this invention is the fabrication of a susceptor or anodefrom a plexus of refractory metals, the plexuses comprising a series ofintertwisted helical convolutions of the metal.

An advantage of this invention is that the anode or susceptor formed ofa foraminous plexuses can have a longer life and be stronger afterheating then similar elements fabricated of sheet metal or machined fromheavy stock.

Many other objects, features and advantages of this invention willbecome manifest to those conversant with the art upon making referenceto the detailed description which follows and the accompanying sheets ofdrawings in which preferred embodiments of susceptors or anodes of arefractory metal, foraminous plexus are shown and described and whereinthe principles of the present invention are incorporated by way ofillustrative examples. Of these drawings:

FIGURE 1 is a perspective view of the preferred embodiment of the anodeor susceptor.

FIGURE 2 is an enlarged fragmentary view of the helical intertwistedwires which form one embodiment of the anode or susceptor shown inFIGURE 1. These wires are joined together at their ends by a weld whichis schematically illustrated.

FIGURE 3 is an enlarged fragmentary view of the helical intertwistedwires which form another embodiment of the anode or susceptor shown inFIGURE 1.

FIGURE 4 is a cross-sectional view of our susceptor disposed within anelectromagnetic-type furnace.

FIGURE 5 is a cross-sectional view of our anode disposed within a hightemperature electric discharge furnace.

FIGURE 6 is a cross-sectional view of an embodiment of the invention inwhich the plexus is embedded in a refractory metal coating.

In the preferred embodiment shown in FIG. 1, the anode or susceptorelement according to our invention can easily be fabricated by forming aplexus 1 of a series of intertwisted, individual wire helixes. Thehelixes may be formed of any of the usual refractory metals such asmolybdenum, columbium, tantalum, rhenium or preferably tungsten.Additionally, alloys of such metals having requisite melting points alsohave applicability in some cases. The wire diameter ordinarily should beabout 0.010 to 0.125" since such wire sizes offer optimumcharacteristics in a furnace. Below about 0.10", the element fabricatedof these metals will volatilize too readily when heated due to the largesurface area and hence, the life of the element will be drasticallyreduced. Above about 0.125" the wire will be difficult to work and coil.The thickness of the plexus will vary depending upon the internaldiameter of the helix together with the diameter of the Wire. It isdesirable for most applications of the element to form the helicalconvolutions on mandrels having diameters of about 0.025 to 0.500".While the upper limit may be increased to suit individual furnace designrequirements, it is generally not feasible to go below the lower limitsstated because the wire which will have to be used will be too fine tomake an efiicient element having reasonable life. Pitch of theindividual convolutions can vary from about 300% (that is, the spacingbetween the turns equaling slightly more than two times the diameter ofthe wire) to about 1000% or even greater. It is apparent however that atthe upper limit, the wires must have sufiicient pitch to allow forintertwisting of several of the convolutions together. Preferably formost applications, we use a pitch of about 300% so that a tight plexusis formed. Now we have stated that fine Wire should not be used becauseit would reduce the life of the element, however it must be pointed outthat when the plexus is flame sprayed with the suitable material, evenfiner wires may be used since internal strength in such cases is notsuch an important prerequisite.

Intertwisting of the wire helixes may be performed easily by disposingone wire in a jig and then rotating a second wire helix into the turnsof the first. A third wire helix is then intertwisted in the turns ofthe second helix and the operation is repeated again and again until aplexus of the desired width and length is attained. Although we show aconfiguration having two convolutions intertwisted in each otherconvolution, many other variations can be used also. For example, thesingle wire shown can be doubled and possibly tripled and themultistranded wire wrapped into helical convolutions of the desiredpitch on a mandrel. Other modifications include insertion of stranded orstraight wire into the turn abutments of the convolutions to affordadditional strength. And yet, another method of intertwisting wireinvolves placing the convolutions of one coil into the interdigitalspaces between the convolutions of another coil and then joining theconvolutions together by threading a straight wire through the twoconvolutions, which process can be repeated until a plexus of a desiredsize is obtained.

Because in the prepared embodiment, the plexus will have to be shapedafter it is formed, it is generally desirable to use coils which havenot been stress-relieved. Such coils have not yet been treated to setthe crystal structure and hence may be bent and shaped as desiredwhereas stress-relieved coils are brittle. After the finalconfigurations of the element are made, the entire unit may then beheated to set the crystal structure.

When a plexus of requisite size is fabricated, it is then ready to beformed into the desired shape. Preferably, the coils of the plexus arebent into a generally cylindrical shape on a plane substantiallyperpendicular to their axes. When the desired shape is obtained, and ifa regular, continuous shape is desired, a weld is formed by conventionalheliarc methods at the edges of the plexus and on the distal ends of theindividual wire helixes.

In the applications of the anodes or susceptors, a generally cylindricalshape is most desirable. Such shapes are preferably cylindrical althoughrectangular, polygonal or irregular shapes may be fabricated easily. Thelatter rectangular, polygonal or irregular shapes have particularapplication where the article which is to be heated has similarrectangular, polygonal or irregular shapes. As an example, the advantageof heating a rectangular article of work with a rectangular element isthat uniform spacing between the article and the element is obtained.While a cylindrical shape might be practical for heating most articlesor work which are small in comparison to the diameter of the element orfor heating large cylindrical objects, the relationship is not alwayspractical when heating large pieces of work of shapes different than theelement. In these cases the other noncylindrical shapes should be used.

For some particular applications it may be desirable to heat one sectionof work to a higher temperature than the other adjacent sections. Suchtemperature differentials may be easily attained 'by shaping the anodeor susceptor so that a section thereof would be closer to the article orwork than the remainder of the element. Due to greater proximity to thework, greater heating will be attained at the point which is nearest tothe element than at adjacent points which are more distant.

Referring now to the detailed view of FIGURE 2, it will be seen that thecoils, 3, 4, 5, are intertwisted in such a manner that the upper coil 3is intertwisted in the intermediate coil 4 which in turn is intertwistedin the lower coil 5. In the embodiment shown, the turns of the coils arespaced fairly closely together (300% pitch) in order to form a denseforaminous plexus.

In order to join the ends of the cylinder together and form a seam, wehave previously indicated that a heliarc weld also may be used, howeverother means of securing the ends together such as mechanical clampingmay also have applicability so long as such means can withstand thetemperatures to which the element will be raised without appreciablewarping, distortion or alteration of the electrical characteristics.

While a seam is the preferred method of forming the plexus into agenerally cylindrical shape, we may also obtain this shape byintertwisting a helical convolution of refractory metal Wire similar tothe adjacent helical convolutions 8 and 9 into the first and last coils6 and 7 of the plexus as shown in FIGURE 3.

Sometimes it is desirable to use a basket-type element particularlywhere there are many small articles of work to be heated together. Theseelements may be easily formed by placing a bottom on the cylindricalelement either 'by heliarc welding a flat, round plexus to the endthereof or using other suitable means of attachment. Of course, evensheet tungsten may be used, however sheets on the bottom suffer from thedifficulties previously indicated. They will tend to warp after heatingand become brittle.

Frequently when ruggedness is a critical factor, such as would be thecase if the element is to be raised to elevated temperatures quitefrequently and possibly for short durations of time, it may be desirableto heliarc weld a reinforcing rim (not shown) around the edges. The rimmay be simply a tungsten flux if tungsten is used as the elementmaterial, or possibly, it may be fabricated of a ring of suitablerefractory metal.

As we have indicated previously, the plexus 52 forming the anode orsusceptor can be metalized by spraying a molten metal onto the surfaceto form a coating 51 as shown in FIG. 6. When using a tungsten element,it is generally preferred to coat tungsten thereupon. Other refractorymetal plexuses can use similar refractory metal coatings. The processinvolves melting the metal in a flame and atomizing it by a blast ofcompressed air into a fine spray. This spray builds up onto the surfaceto form a solid metal coating. Because the molten metal is accompaniedby a large blast of air, the object being sprayed is not heated sharply.When the element of our invention is metalized, the flexibility is lost,however strength is markedly increased. Even when a furnace temperatureof 3000 C. is used, there is such a quantity of tungsten present thatthe life will be very long. Furthermore just as reinforced concrete hasa greater strength than concrete without reinforcement rods, so too is ametalized heating element formed upon a foraminous plexus.

Referring now to FIGURE 4 of the drawing, a typical induction furnace isshown wherein the susceptor of our invention is illustrated. Theprinciple equipment of an induction heater can be a coil or wire throughwhich an alternating current flows. The alternating current induces eddycurrents in any metal that is either located within the coil or closelysurrounds the coil. The frequency ranges between 60 c.p.s. and 3magacycles and even higher in certain special applications. Heat isgenerated in the metallic article or work by induction regardless of thecomposition of any non-metallic materials which are disposed between thearticle of work and the coil. The coils are generally wound fromflattened copper pipes through which water is circulated.

In the furnace, a flow of inert and/ or reducing gas can be passedthrough the inlet port 10 over the members within the furnace which areto be heated and removed from an outlet port 12. In cases where a vacuumis to be maintained, the reduction of pressure can be attained bywithdrawing gases through the outlet port 12 while the inlet port 10 isclosed. The furnace 15 is lined with insulation 14 to reduce heat lossesand when desired, a series of heat shields (not shown) can be disposedimmediately inside of the insulation .14.

An internally water cooled helical coil 16 is attached to the source ofcurrent (not shown) by conventional techniques and the susceptor 18according to our invention is disposed inside of the helical coils 16.As shown, the susceptor can have a generally cylindrical shape and canbe fabricated according to the description indicated previously.Suitable supports (not shown) are provided within the furnace 15 so asto hold the susceptor 18 axially aligned within the helical, watercooled coil 16. An article of work (not shown) can be supported,suspended or disposed within the susceptor 18, as is conventional in theart.

While the preferred embodiment of our invention involves using theforaminous plexus which is bent on a plane perpendicular to the axes ofthe individual helical convolutions which are used to make it, it isapparent that other modifications may be used equally well. Frequentlyit is possible to use convolutions which need not be shaped until afterthe fabrication of the plexus. Such arrangement can easily be made bybending the plexus in a plane substantially parallel to the individualconvolutions and then joining the ends together with a similar wire coilwhich will act as a retainer. In this manner an entirely cylindricalelement can be formed without the necessity of welding.

The embodiment of our invention in which the heating element is used asan anode is shown in FIG. 5. In this embodiment, a furnace 20 whichincludes a layer of insulation 21 is used to surround the entire heatingunit so as to contain the heat therein. Supported upon three buss bars,22 and 23 (the third buss bar is not shown in this cross section) is acathode which may be formed according to our co-pending application Ser.No. 219,404, filed Aug. 27, 1962 entitled Electrical Heating Element andassigned to the same assignee as the instant application, now UnitedStates Patent 3,178,665. As therein described the heating element cancomprise three grates 24, 25, and 38. The anode is preferably assembledin a substantially cylindrical form and can be connected together at oneend by an outer conductor ring 27 and an inner conductor ring 28.Preferably the attachment of the grates to the conductor rings is madeby heliarc welding the rings together on their lowermost extremity.

At the other end of the cathode are supporting, electrically conductingarms 29 and 30 which extend radially upwardly from the heating element.These supporting arms 29 and 30 are adapted to be disposed withinelectrically conducting buss bars 22 and 23. Since we prefer to make thesupporting arms of at least two portions and preferably four asdescribed in the abovementioned application, we have found that it isfrequently desirable to tie the portions together with windings 32 and33. Such windings are preferable since they afford radiant heatdissipation at the outward extremities of the arms and help to preventoverheating of the buss bars. Disposed inside of the grates 24 and 25,at the upper end thereof, are inner support segments 34 and 35 which arejoined to the grate 24 and 25 and in turn to the support arms 29 and 30by a flux-type weld 36 and 37. The grate 38 and the rest of itsstructure is similar to that of either grates 24 and 25.

Surrounding the cathode are a series of heat shields 40 which may varyin number depending upon the heat to which the furnace is to be raised.These heat shields 40 are generally supported upon a rod or othersuitable support device 42 which may be affixed to the refractory liner43. A water cooling system 44 can be advantageously used to keep thetemperature of the furnace within reasonable limits and to preventlocalized overheating.

The anode '45 of our invention can be conveniently disposed within thecathode in such a manner that the axis thereof substantially coincideswith the axes of the anode. Any articles of work which are to be heatedcan be supported, suspended or disposed within the anode 45 in a mannerwhich is conventional in the art. The anode itself can be convenientlysupported upon a stand 46 which is in turn supported upon refractorystruts 47. The anode is placed in the electrical circuit -by passingcurrent to the stand 46 through means well known in the art. Disposedbeneath the anode are a series of heat shields 48 which radiate heatupwardly into the furnace.

While there is shown and described herein certain specific structureembodying our invention, it will be manifest to those skilled in the artthat various modifications and rearrangements of the parts can be madewithout departing from the spirit and scope of the underlying conceptand that the same is not to be limited to the particular fOlIIIlS of theinvention herein shown and described, except insofar as indicated by thescope of the appended claims.

As our invention we claim:

1. An induction heating furnace having a water cooled helical coildisposed therein, said coil being attached to a source of current, theimprovement which comprises: a susceptor disposed inside of said coiland spaced therefrom, said susceptor being a generally cylindricalplexus formed of a multiplicity of intertwisted helical convolutions ofrefractory metal wires.

2. The induction heating furnace according to claim 1 wherein thesusceptor is formed in the generally cylindrical shape by joiningtogether the distal ends of the multiplicity of intertwisted helicalconvolutions.

3. The induction heating furnace according to claim 1 wherein thegenerally cylindrical shape of the susceptor is formed by intertwistinga helical convolution of refractory metal wire similar to the helicalconvolutions in the plexus into the first and last convolutions thereof.

4. The induction heating furnace according to claim 1 wherein therefractory metal wires have a diameter of about 0.010 to- 0.125 inch.

5. The induction heating furnace according to claim 1 wherein a flamesprayed coating of a refractory metal is disposed upon the outersurfaces of the plexus.

6. The induction heating furnace according to claim 1 wherein therefractory metal is tungsten.

References Cited UNITED STATES PATENTS 223,262 10/ 1880 Wakeman 245-53,036,888 5/1962 Lowe 219-1049 X 3,172,002 3/1965 Johnson et al 3133483,210,455 10/1965 Sedlatschek 1327 3,274,374 9/1966 Matheson et a1.219426 3,350,494 10/1967 Kunitsky et a1 13-27 RICHARD M. WOOD, PrimaryExaminer.

L. H. BENDER, Assistant Examiner.

US. Cl. X.R. 1327

